Spark ignited internal combustion engines can encounter abnormal combustion events, such as knock, under high load operation, especially when operating with lower octane fuels. In-cylinder temperatures may provide a significant indication as to whether a cylinder is likely to knock or not. In particular, hotter cylinders may have a larger tendency to knock. Further, based on an engine configuration, such as the location of cooling passages and engine packaging constraints, some cylinders may be more prone to knock that others. Knock may be addressed by retarding spark timing of engine cylinders. While spark retard improves knock, it results in lower engine torque output and reduced fuel economy.
In recent years, spark ignited combustion engines have been configured to operate with a variable number of active or deactivated cylinders to increase fuel economy, while optionally maintaining the overall exhaust mixture air-fuel ratio about stoichiometry. Such engines can vary the effective displacement of the engine by skipping the delivery of fuel to certain cylinders in an indexed cylinder firing pattern, also referred to as a “skip-fire” pattern. For example, as shown by Tripathi et al. in U.S. Pat. No. 8,651,091, an engine fuel controller may continuously rotate which particular cylinders are fueled, which cylinders are skipped, and how many cylinders events the pattern is continued for. By skipping feel delivery to selected cylinders, the active cylinders can be operated near their optimum efficiency, increasing the overall operating efficiency of the engine.
The inventors herein have recognized that the amount of spark retard applied for knock control (and thereby the fuel penalty associated with knock control) can be reduced by leveraging individual cylinder valve deactivation mechanisms. In particular, the fast responding cylinder valve deactivation mechanisms of skip-fire engines may be used to transiently deactivate cylinders with a higher knocking rate, the identity of the deactivated cylinder varied as the knock pattern of the cylinders change with engine operating conditions. One example approach includes, deactivating individual cylinder valve mechanisms according to a cylinder pattern selected based on a knock value of each engine cylinder. In this way, knock may be controlled with the use of less spark retard.
As an example, if a cylinder has been knocking frequently, a controller may selectively deactivate the given cylinder for one or more combustion events. The knocking cylinder may be deactivated by deactivating intake and exhaust valve operation of the cylinder while also disabling fuel and spark to the cylinder. For example, the cylinder may be deactivated when a knocking rate or occurrence in the cylinder exceeds a threshold. Alternatively, with each knock event, spark timing may be retarded away from a borderline value. Then, as the knock frequency increases and spark retard exceeds beyond a threshold (e.g., becomes borderline limited), the cylinder may be deactivated. Due to the deactivation, the cylinder may start cooling, reducing its propensity for further knock events. When the in-cylinder temperature falls below a threshold temperature, the cylinder may be reactivated. The cylinder may then be operated with less spark retard, for example, spark timing of the reactivated cylinder may be advanced towards MBT. Alternatively, as the engine cylinder cools and another engine cylinder becomes concurrently hotter, and more borderline spark limited, the more borderline limited cylinder may be deactivated while the cooled cylinder may be reactivated.
In this way, by adjusting a pattern of cylinder deactivation based on cylinder knock occurrence, engine knock can be controlled with the use of less spark retard. As a result, the fuel penalty associated with knock control can be reduced. By deactivating cylinders that have higher knock incidence until they are sufficiently cool, and then reactivating them, the cylinders may be operated with spark closer to MBT. By continually varying the pattern of deactivated/active cylinders such that cylinders are deactivated when they knock more and reactivated when they less, cylinder temperatures may be controlled reducing the propensity for further knock events. By operating active cylinders with spark advanced from borderline towards MBT, the engine may be operated with less knock occurrence and with higher fuel economy.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.